I gots to know?

Why is an 18 wheeler called a "Semi"? Just something I have been curious about. I think about the term semi and think something like semi-retired where someone is retired and yet still works some. Or semi-pro...not quite a pro player but close. How does a semi-truck relate to this. Thanks, Gary.

I have been told as a kid that the full term was "semi-trailer" and
was so called because instead of being entirely towed behind the
tractor it was partially on top of it. There was also an old friend of
my mother who was sometimes referred to as the "Semi" which I
later found out was short for semi-family but I didn't understand
that at the time and couldn't see any resemblance between her and
a truck.
Zach

A truck and trailer is usually a 6 wheeled truck pulling a trailer with 4 wheels usually a steering axle and a non steering asle. A semi is a truck with either 4 or 6 wheels and pulling a trailer with 1 or 2 axles on the rear of it. That's my interpertation of it.

Here's what I found when I Googled it,but in my little world,a trailer has always been something with no wheels in front. Seems to me it should mean semi-wagon if it means anything. 'Course that brings up the whole "station wagon" thing doesn't it?

"Semi truck or semi is short for semi-trailer truck. It describes a truck consisting of a tractor and a half-trailer. The trailer is a half-trailer because it does not have wheels both front and back, but only in back. The front is supported by the tractor."

Well, I can't quote it from memory, but the FMCSA defines it like this- A full trailer has a steering axle in the front and none of the weight rests on the power unit. A semi-trailer has the front end resting on a 5th wheel on the power unit. That handles the "semi" part as far as the trailer. Why people call a tractor and trailer, a tractor being a power unit on which none of the actual load is carried but which serves as just the power unit vs a truck which carries at least part of the actual load, a "semi" I don't really know. Slang I guess, short for tractor ad semi-trailer...which IS quite a mouthful when you come down to it.

I always assumed semi meant a two-piece unit, vs. a straight truck. Then how about pronunciation? US sem(eye) vs. Canadian sem(me).

(quoted from post at 16:03:31 06/17/14) Well, I can't quote it from memory, but the FMCSA defines it like this- A full trailer has a steering axle in the front and none of the weight rests on the power unit. [b:54bbe4f169]A semi-trailer has the front end resting on a 5th wheel on the power unit[/b:54bbe4f169]. That handles the "semi" part as far as the trailer. Why people call a tractor and trailer, a tractor being a power unit on which none of the actual load is carried but which serves as just the power unit vs a truck which carries at least part of the actual load, a "semi" I don't really know. Slang I guess, short for tractor ad semi-trailer...which IS quite a mouthful when you come down to it.

Click to expand...

ell by your definition that means a 1 ton truck picup with goose neck stock trailer should classify as a semi-trailer :lol:

so where does that put us when you look at doubles.

ie.. trator.. trailer, and then the 2nd trailer that DOES have wheels in front as well as back?

Well,the dolly makes it a wagon doesn't it? That's my story and I'm sticking to it until some better explanation comes along. lol

Another Google answer.

"It's mostly just a nomenclature thing although I have usually seen them referred to as wagons when they had steerable wheels up front regardless of the type of wheels and trailers when they had wheels only in the rear"

welcome to Australia lol.

i have always heard of trailers with front steering axles as wagons too..

I'd agree.

I drove a semi fur 30 years and didn't think aboot that.
I had more serious thangs on my mind, like why do we drive on the parkway and park in the drive way?
Why are the roads called freeways, and it cost a lot to drive on them?
Also of question here in Indiana South Bend is up north, North Vernon is down south and French Lick is a disappointment,
You have to go north to git to South Dakota, and you have to go south to git to North Carolina, to git to West Virginia one has to go east, if going to East St.Louis one has to travel west.
You see I had a lot of thangs to ponder vill engineering a semi along the roads.

A semi is a trailer that carries only a portion of the weight. The connection point is above and in front of the truck (tractor) rear axle so the truck carries a large portion of the weight.

When you add a dolly (steering axle; such as in doubles) to the front of a semi trailer it becomes a full trailer.

With the invention of goose necks and smaller one ton semi trailers "semi" has come to insinuate large commercial units over the years to distinguish between the two.

OK,now we seem to know what a semi trailer is.Now what is a semi truck,or a semi tractor??? LOL

It's only half a truck. There's no bed on it. Useless in itself. That's my theory anyway.

the government"s word for an 18 wheeler

Sorry, this got longer than expected.
See John in La's reply above for semi-trailer. That covers single trailer, or the lead trailer in a doubles or triples set, as the tractor carries part of the load. For the second trailer in doubles & third in triples, they are considered as full trailers, as the towing unit doesn't carry any of their weight.
Now for truck & trailer vs semi tractor/trailer.
Common useage for truck & trailer is your contractor dump towing a pup. In this case the truck can carry a load by itself. The pup is considered a full trailer as it carries its load by itself. The only part of its weight that the truck carries is the weight of the towbar/hitch.
A semi tractor does not carry a load by itself, needs a trailer behind it.
Years ago there were some rigs out there that had a small platform behind the cab, then the 5th wheel hitch for the semi-trailer. Then it was technically a truck & trailer, could pile a few boxes or crates on it, & was a way to get around overall length laws in some states. Some even had insulated heated/cooled boxes for perishable goods. That was called a "drom box", short for dromedary, it looked like the hump on a camel.
Wagon vs trailer: in my part of the world a wagon was a farm implement mostly used in the field, or hauling goods to market in town. A trailer was an on-road vehicle for hauling everything else.
But go east across the Big Pond, everything that hauls freight, even railroad box cars, are called wagons.
I know, clear as mud, but that covers the ground.
Willie diesel pilot-retired

Like the description of a "semi-aerobatic" sailplane as

"That means that half the sailplane is fully aerobatic"

According to regulations about here it just might.

Depends on whether the ball is in front of or behind the rear axle line.

So a B train, or 'super Bee' like the ones we see in Canada should be called a semi-semi? Jim

(quoted from post at 00:07:16 06/18/14) So a B train, or 'super Bee' like the ones we see in Canada should be called a semi-semi? Jim

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Nope, a B train has it's own distinct definition spelled out in the FMCSA rules and regs. It's always a B train.

(quoted from post at 20:14:08 06/17/14)

(quoted from post at 16:03:31 06/17/14) Well, I can't quote it from memory, but the FMCSA defines it like this- A full trailer has a steering axle in the front and none of the weight rests on the power unit. [b:d6a2870185]A semi-trailer has the front end resting on a 5th wheel on the power unit[/b:d6a2870185]. That handles the "semi" part as far as the trailer. Why people call a tractor and trailer, a tractor being a power unit on which none of the actual load is carried but which serves as just the power unit vs a truck which carries at least part of the actual load, a "semi" I don't really know. Slang I guess, short for tractor ad semi-trailer...which IS quite a mouthful when you come down to it.

Click to expand...

ell by your definition that means a 1 ton truck picup with goose neck stock trailer should classify as a semi-trailer :lol:

Click to expand...

Nope, the pickup is DESIGNED to carry a load on the power unit which makes it a truck. A tractor is designed to pull the load, not carry any of it, it's strictly a power unit. Yes, you can split hairs, but the pickup is always going to be a truck, even if it's towing cattle or cars or something else that couldn't possibly fit in the box.

Also, a gooseneck uses a ball hitch vs a 5th wheel. I forget the distinctions, but there are some for that too.

Hey, I don't make the rules, I just read them! :lol: